1. Field of the Invention
The present invention relates generally to processing substrates, and more particularly to methods and apparatuses for monitoring and controlling removal rate for substrate processing systems.
2. Description of the Related Art
Integrated circuits are typically formed on substrates, particularly silicon wafers, by the sequential deposition of conductive, semiconductive or insulative layers. After each layer is deposited, a layer may be etched to create circuitry features. As series of layers are sequentially deposited and etched, the outer or uppermost surface of the substrate, i.e., the exposed surface of the substrate, becomes increasingly nonplanar. This nonplanar surface presents problems in the photolithographic steps of the integrated circuit fabrication process. Therefore, there is a need to periodically planarize the substrate surface. In addition, planarization is often needed to remove a filler layer until an underlying stop layer is exposed, or to create a layer with a defined thickness.
Chemical mechanical processing (CMP) is one accepted method of planarization. This planarization method typically requires that the substrate be mounted on a carrier or a polishing head. Conventionally, the exposed surface of the substrate is placed against a rotating polishing pad, although a linear belt or other polishing surface can be used. The polishing pad may be either a “standard” pad or a fixed-abrasive pad. A standard pad has a durable roughened surface, whereas a fixed-abrasive pad has abrasive particles held in a containment media. The carrier head provides a controllable load on the substrate to push it against the polishing pad. A polishing slurry, including at least one chemically-reactive agent, and abrasive particles if a standard pad is used, is supplied to the surface of the polishing pad (also, some polishing processes use a “nonabrasive” process) in a CMP process.
An important step in a CMP process is determining whether the polishing process is complete, i.e., whether a substrate layer has been planarized to a desired flatness or thickness or whether an underlying layer has been exposed. If an excessive amount of material is removed (overpolishing), the substrate is rendered unusable. On the other hand, if an insufficient amount of material is removed (underpolishing), the substrate must be reloaded into a CMP apparatus for further processing.
Therefore, the removal rate of CMP apparatuses is an important variable to monitor. Various methods are used to measure the layer thickness before and after a polishing step in order to calculate the removal rate of a CMP apparatus. Removal rate of a CMP apparatus is generally monitored in order to schedule a sufficient processing time for each CMP step. For example, a spectrometer, such as the NovaScan 210, manufactured by the Nova Corporation of Israel, can be used as an in-line metrology device to measure the thickness of one or more layers in the substrate before and after a process step in a polishing station in order to calculate the removal rate.
As illustrated in FIG. 1, a substrate processing system 100 includes a CMP polisher 22, a wet robot 24, a cleaner 26, a factory interface module 28, and an in-line metrology station 30, which includes a metrology device 60. In a standard process, substrates 11 are transported to the substrate processing system 100 in cassettes 12, and are extracted from the cassettes 12 by a robot 18 in the factory interface module 28 for transport to the polisher 22 or cleaner 26 or metrology device 60. Illustratively, the polisher 22 includes three polishing stations 25c, 25b, and 25a, and a transfer station 27.
Each processing station may perform a different function. As an example, a first polishing station 25c may be provided for bulk material removal through a first CMP process, a second polishing station 25b may be provided for residual material removal through a second CMP process, and a third polishing station 25a may be provided for barrier layer material removal through a third CMP process. At each of the three polishing stations 25c, 25b, and 25a, a substrate 11 undergoes a polishing process defined by processing time based on a removal rate.
It is well known in the art that a removal rate for each process may vary over time due to factors such as: pad wear, variation in slurry composition, variations in the composition of the layers being removed, and other such factors. FIG. 2 illustrates exemplary variation in the removal rate of blanket (or calibration) substrates over a number of processing runs in a polishing station 25. As illustrated, after 500 processes, the polishing rate is approximately 350 Angstroms per minute. Subsequently, the polishing rate drops well below 350 Angstroms per minute after 1100 processing runs. Unless processing time is adjusted accordingly, the variations in removal rate will lead to non-uniform substrate thicknesses.
Therefore, to monitor removal rate, in conventional processing systems, a number of substrates 11 are periodically transferred into a metrology device 60 for thickness measurements before and after processing by the polishing stations 25c, 25b and 25a. A removal rate, calculated based on the measured thicknesses before and after processing, may then be used to adjust the processing time (duration) of one or more of the polishing stations 25 in CMP polisher 22. Overall operations, including adjusting polishing times, may be controlled by controller 32, which may include one or more programmable digital computers executing any appropriate control software. The controller 32 may obtain thickness measurements from the metrology device 60, calculate a removal rate, and adjusts processing times for one or more of the polishing stations 25c, 25b, and 25a, accordingly.
Although measuring removal rate is important to the overall processing of substrates, it adds to the overall processing time, since it requires the transfer of substrates to metrology device 60 and thus adversely affects the system throughput (number of substrates per hour). Further, an in-line metrology tool adds significantly to the overall cost of the system.
Therefore, there is a need for an improved method and apparatus for measuring removal rate in a CMP system.